The works of Francis Maitland Balfour, Volume 2 (of 4)

by Francis M. Balfour

Fig. 7. Section through a small part of the surface of an ovum of an immature female of Scyllium Canicula.

fe. Follicular epithelium. vt. Vitelline membrane. Zn. Zona radiata. yk. Yolk with protoplasmic network.

The chorion is often ornamented with various processes, etc. It is in many cases doubtful whether a particular membrane is a chorion or a vitelline membrane.

All the membranes which surround the ovum may be provided with a special aperture known as the micropyle. A micropyle is by no means found in the majority of types, and there is no homology between the various apertures so named. Micropyles have two functions, either to assist in the nutrition of the ovum during its development, or (2) to permit the entrance of the spermatozoa. The two functions may in some cases coexist. Micropyles of the first class are developed at the point of attachment of the ovum to the wall of the ovary or to its follicle. Good examples of this kind of micropyle are afforded by the Lamellibranchiata ([fig. 12]), Holothuria, and many Annelida (Polynoe, etc.). The micropyle of the Lamellibranchiata (p. [37]) probably serves also to admit the spermatozoa. The second type of micropyle is found in many Insecta, Teleostei, etc.

General Bibliography of the Ovum.

(1) Ed. van Beneden. “Recherches sur la composition et la signification de l’œuf,” etc. Mém. cour. d. l’Acad. roy. des Sciences de Belgique, Vol. XXXIV. 1870.
(2) R. Leuckart. Artikel “Zeugung,” R. Wagner’s Handwörterbuch d. Physiologie, Vol. IV. 1853.
(3) Fr. Leydig. “Die Dotterfurchung nach ihrem Vorkommen in d. Thierwelt u. n. ihrer Bedeutung.” Oken, Isis, 1848.
(4) Ludwig. “Ueber d. Eibildung im Thierreiche.” Arbeiten a. d. zool.-zoot. Institut Würzburg, Vol. I. 1874[15].
(5) Allen Thomson. Article “Ovum” in Todd’s Cyclopædia of Anatomy and Physiology, Vol. V. 1859.
(6) W. Waldeyer. Eierstock u. Ei. Leipzig, 1870.

Special History of the Ovum in different types.

Cœlenterata.

(7) Ed. van Beneden. “De la distinction originelle d. testicule et de l’ovaire.” Bull. Acad. roy. Belgique, 3^e série, Vol. XXXVII. 1874.
(8) R. and O. Hertwig. Der Organismus d. Medusen. Jena, 1878.
(9) N. Kleinenberg. Hydra. Leipzig, 1872.

Amongst the Cœlenterata the ova are developed in imperfectly specialized organs, which are situated in various parts of the body, for the most part in the space between the epiblast and the hypoblast.